The present invention relates to a device and method for measuring the electrical properties of a dielectric film at frequencies in the Gigahertz range.
The progression towards VLSI chips with an increasing number of transistors of smaller and smaller dimensions has resulted in faster and faster signal transmission within such chips. This increased circuit speed has created a need for a method of interconnecting such chips without losing the benefit of this speed in the interconnecting lines. Thus, scientists have developed high density interconnects for use in interconnecting two or more product chips without suffering severe speed degradation. In the foregoing method an insulating substrate which is typically a silicon wafer is coated with a conducting material such as aluminum to form a ground plane over the wafer surface. A dielectric layer is deposited over the aluminum and then another aluminum layer deposited over the dielectric. The final aluminum layer is patterned to form bonding pads for the product chips and interconnecting lines. The electrical signal transmission properties depend critically on the performance of the dielectric. The relative dielectric constant and the radio frequency energy dissipation of the dielectric directly affect the speed and attenuation of a signal travelling along an interconnect line. The properties of the dielectric, in turn, depend on fabrication and environmental variables such as the cure profile and humidity. Consequently, a measurement circuit and procedure for monitoring the fabrication of high density interconnect circuits is needed.
At frequencies below 100 Megahertz there are many methods available to measure the electrical properties of a dielectric material. These methods are based on using the dielectric as the insulating layer in a capacitor structure. However, at Gigahertz (GHz) frequencies, a capacitor with a capacitance large enough to make measurement errors negligible is so reactive that it simply reflects all energy and yields no data on the value of the capacitance.
Other methods exist for measuring the properties of dielectrics in bulk form at GHz frequencies. These methods generally consist of inserting the bulk material into a wave guide structure and measuring the effect of the electrical behaviour of having the dielectric within the wave guide relative to having air in the wave guide. Two standard methods of measuring the dielectric properties are described in military specification MIL-P-13949F, Appendix I and Appendix II. The method of Appendix II is only applicable at frequencies below 100 Megahertz. The method of Appendix I uses a circuit fabricated using the dielectric material as the insulating layer, the critical part of the circuit being a half-wave resonator into which a small quantity of energy must be coupled in order to stimulate the resonance. A small portion of the energy stored in the resonating part of the circuit is tapped off and measured. For accurate measurements of the dielectric properties, the resonator should resonate with as little outside influence or loading as possible. The method of coupling energy into and out of the resonating part of the circuit is by means of a small non-conducting gap to the input and output connections. The difficulties arise if the dielectric film is so thin that the dimensions of the gap required for coupling are too small to reasonably fabricate. This is the case if the film thickness is on the order of 10 micrometers.
Another test is described by N.K. Das et al in a paper entitled "Two Methods for the Measurement of Substrate Dielectric Constant" which was published in the I.E.E.E. Transactions on Microwave Theory and Techniques, Volume MTT-35, No. 7, July, 1987, p. 636. In this article there is disclosed the characterization and comparison of two similar transmission lines. The two transmission lines are fabricated with the dielectric material as the insulating layer and with the transmission line dimensions except for length being as similar to each other as possible. Each of the transmission lines is mounted with separate input and output connections to the measurement equipment, and the electrical length and attenuation is then measured. The results are compared and the effects of the test jig should cancel, leaving only the electrical behaviour of the extra portion of the longer transmission line. The difficulty with this method is that errors are introduced by the inability to make the electrical connections to the transmission lines behave the same at GHz frequencies for both the short and the long line.
If the dielectric material to be characterized is in thin film form, with a thickness of less than 40 micrometers, and if the electrical properties of the material need to be determined at GHz frequencies, none of these measurement methods are suitable. Additionally, if the dielectric material and a production electrical circuit using the dielectric material are to be characterized, a test method which can be applied using the same substrate and structures similar to those of the production circuit is more useful than one that requires fabrication of a separate test sample.
Accordingly, it is an object of the present invention to produce an improved test device and method for measuring and determining the electrical properties of a thin film of dielectric material.
It is yet a further object of the present invention to produce a device of similar dimensions and on the same substrate as the high density interconnect structure in order to facilitate the estimation of the electrical properties of the product by testing only the test device.